code: def test_backward_gpu(self):
      self.check_backward(
          (cuda.to_gpu(self.diag), cuda.to_gpu(self.non_diag)),
          cuda.to_gpu(self.gy),
      )
  
file_path: chainerrl/tests/functions_tests/test_lower_triangular_matrix.py
imports: import unittest
  import numpy
  import chainer
  from chainer import cuda
  from chainer import gradient_check
  from chainer import testing
  from chainer.testing import attr
  from chainerrl.functions.lower_triangular_matrix import lower_triangular_matrix
  from chainerrl.functions.lower_triangular_matrix import LowerTriangularMatrix
label_desc: Use image node_id 7 for calling the TestLowerTriangularMatrix obj's underlying member method code with example usage: obj.test_backward_gpu() without return types